Field of the Invention
This invention relates to recovery of heat and water from aqueous waste streams, and more particularly to improved heat and water recovery in extraction of bitumen from tar sands extraction.
Tar sands (also known as oil sands and bituminous sands) are sand deposits which are impregnated with dense, viscous petroleum. One method for recovering bitumen from such tar sands is the so-called hot water extraction process. In such a process, the tar sand feed is heated and mixed with water to form a pulp, with such pulp being heated by live steam. The pulp is screened and introduced into separation cells which function as two settlers, one on top of the other. The lower settler settles sand down, and the upper settler settles bitumen up: i.e. floats the bitumen. The bulk of the sand in the feed is removed from the bottom of the separation cell as tailings. A major portion of the feed bitumen floats to the surface of the separation cell and is removed as froth. A middlings stream consisting mostly of water, but with some suspended fine mineral and bitumen particles, is the third stream removed from the separation cell. A portion of the middlings may be returned for mixing in the extraction drum in order to dilute the separation cell feed properly for pumping. The balance of the middlings is called the drag stream. Such drag stream is withdrawn from the separation cell to be rejected after processing in the scavenger cells. The drag stream is primarily required as a purge in order to control the fines concentration in the middlings. The drag stream is treated in scavenger cells in order to recover further bitumen. Such scavenging may be accomplished by froth flotation using air, whereby the scavenger froth is combined with the separation cell froth to be further treated and upgraded to synthetic crude oil. Tailings from the scavenger cell are combined with the separation cell tailings stream and go to waste.
The tailings stream from the process contains the bulk of the heat and water that was supplied to meet the process requirements. As a result, such tailings stream represents a heat and water loss.
The present invention is directed to improving heat and water recovery from aqueous waste streams, such as produced in tar sands extraction.